netdev - Re: [PATCH 1/1] bnx2x: Tx barriers and locks

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Date:	Tue, 2 Mar 2010 12:30:33 +0100
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Michael Chan <mchan@...adcom.com>
Cc:	Vladislav Zolotarov <vladz@...adcom.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	Eilon Greenstein <eilong@...adcom.com>,
	Matthew Carlson <mcarlson@...adcom.com>
Subject: Re: [PATCH 1/1] bnx2x: Tx barriers and locks

On Mon, Mar 01, 2010 at 09:59:07AM -0800, Michael Chan wrote:
> > There is still difference between what we have in bnx2x and bnx2/tg3
> > regarding memory barriers in tx_poll/start_xmit code. Mainly we have
> > smp_mb() in bnx2/tg3_tx_avail(), and in bnx2/tg3_tx_int() is smp_mb()
> > not smp_wmb(). I do not see that bnx2x is wrong, but would like to know
> > why there is a difference, maybe bnx2/tg3 should be changed?
> > 
> 
> The memory barrier in tx_int() is to make the tx index update happen
> before the netif_tx_queue_stopped() check.  The barrier is to prevent a
> situation like this:
> 
>     CPU0					CPU1
>     start_xmit()
>     	if (tx_ring_full) {
>     						tx_int()
>     							if (!netif_tx_queue_stopped)
>     		netif_tx_stop_queue()
>     		if (!tx_ring_full)
>     							update_tx_index 
>     			netif_tx_wake_queue()
>     	}
>     
> 
> The update_tx_index code is before the if statement in program order,
> but the CPU and/or compiler can reorder it as shown above. smp_mb() will
> prevent that.  Will smp_wmb() prevent that as well?

No. smp_wmb() affect only write orders on CPU1 performing tx_int(), so
that should be fixed in bnx2x.

Regarding memory barrier in tx_avail(), I don't think it its needed for
anything, except maybe usage at the beginning of start_xmit(), but we can
just remove that like in the patch below. I going to post "official"
patches for tg3, bnx2 and bnx2x, if no nobody has nothing against.

diff --git a/drivers/net/bnx2x_main.c b/drivers/net/bnx2x_main.c
index ed785a3..0f406b8 100644
--- a/drivers/net/bnx2x_main.c
+++ b/drivers/net/bnx2x_main.c
@@ -893,7 +893,6 @@ static inline u16 bnx2x_tx_avail(struct bnx2x_fastpath *fp)
 	u16 prod;
 	u16 cons;
 
-	barrier(); /* Tell compiler that prod and cons can change */
 	prod = fp->tx_bd_prod;
 	cons = fp->tx_bd_cons;
 
@@ -963,9 +962,8 @@ static int bnx2x_tx_int(struct bnx2x_fastpath *fp)
 	 * start_xmit() will miss it and cause the queue to be stopped
 	 * forever.
 	 */
-	smp_wmb();
+	smp_mb();
 
-	/* TBD need a thresh? */
 	if (unlikely(netif_tx_queue_stopped(txq))) {
 		/* Taking tx_lock() is needed to prevent reenabling the queue
 		 * while it's empty. This could have happen if rx_action() gets
@@ -11177,10 +11175,9 @@ static netdev_tx_t bnx2x_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	struct eth_tx_bd *tx_data_bd, *total_pkt_bd = NULL;
 	struct eth_tx_parse_bd *pbd = NULL;
 	u16 pkt_prod, bd_prod;
-	int nbd, fp_index;
+	int nbd, fp_index, i, ret;
 	dma_addr_t mapping;
 	u32 xmit_type = bnx2x_xmit_type(bp, skb);
-	int i;
 	u8 hlen = 0;
 	__le16 pkt_size = 0;
 
@@ -11195,10 +11192,9 @@ static netdev_tx_t bnx2x_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	fp = &bp->fp[fp_index];
 
 	if (unlikely(bnx2x_tx_avail(fp) < (skb_shinfo(skb)->nr_frags + 3))) {
-		fp->eth_q_stats.driver_xoff++;
-		netif_tx_stop_queue(txq);
 		BNX2X_ERR("BUG! Tx ring full when queue awake!\n");
-		return NETDEV_TX_BUSY;
+		ret = NETDEV_TX_BUSY;						
+		goto stop_queue;
 	}
 
 	DP(NETIF_MSG_TX_QUEUED, "SKB: summed %x  protocol %x  protocol(%x,%x)"
@@ -11426,19 +11422,24 @@ static netdev_tx_t bnx2x_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	mmiowb();
 
 	fp->tx_bd_prod += nbd;
-
-	if (unlikely(bnx2x_tx_avail(fp) < MAX_SKB_FRAGS + 3)) {
-		netif_tx_stop_queue(txq);
-		/* We want bnx2x_tx_int to "see" the updated tx_bd_prod
-		   if we put Tx into XOFF state. */
-		smp_mb();
-		fp->eth_q_stats.driver_xoff++;
-		if (bnx2x_tx_avail(fp) >= MAX_SKB_FRAGS + 3)
-			netif_tx_wake_queue(txq);
-	}
 	fp->tx_pkt++;
+	
+	ret = NETDEV_TX_OK;
+	if (unlikely(bnx2x_tx_avail(fp) < MAX_SKB_FRAGS + 3))
+		goto stop_queue;
+
+	return ret;
 
-	return NETDEV_TX_OK;
+stop_queue:
+	netif_tx_stop_queue(txq);
+	/* paired barrier is in bnx2x_tx_int(), update of tx_bd_cons
+	 * have to be visable here, after we XOFF bit setting */
+	smp_mb();
+	fp->eth_q_stats.driver_xoff++;
+	if (bnx2x_tx_avail(fp) >= MAX_SKB_FRAGS + 3)
+		netif_tx_wake_queue(txq);
+	
+	return ret;
 }
 
 /* called with rtnl_lock */
--
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